In well completions and during wellbore plugging and abandonment, it is often necessary to place solid material at different locations in the well. Generally, this is done by pumping a slurry, a fluid mixed with solid material, into the wellbore to the desired location in the subsurface formation. For example, it is often desirable to place a plug in a wellbore to isolate different zones within the formation; as when hydrocarbon bearing zones, that are typically found deeper in the wellbore, are isolated from shallower water bearing zones, to prevent contamination of the aquifer. In this case, concrete, a slurry of cement and gravel, is pumped into the wellbore and permitted to harden.
In other wellbore completion operations, such as gravel packing or fracturing, slurries are pumped under high pressure and at high velocity into the wellbore. In gravel packing, the slurry, which typically consists of gravel in water, is pumped into the wellbore to an angular flow diverter to pack the annulus between the wellbore and the casing with gravel, to prevent the production of formation sand. In fracturing, the slurry includes a proppant, typically sand, that is pumped into the formation to stimulate low-permeability reservoirs and to keep the fracture open. In each of the above cases, the solid material in a slurry is hard, irregularly shaped and may flow at high velocity. Therefore, slurries tend to be highly abrasive, and the machinery that pumps and controls the slurries is subject to abrasive wear and reduced longevity.
Abrasive wear occurs when the particles within the fluid impact on the exposed surfaces of the machinery and impart some of their kinetic energy into the exposed surface. If sufficiently high, the kinetic energy of the impacting particles creates significant tensile residual stress in the exposed surface, below the area of impact. Repeated impacts cause the accumulation of tensile stress in the bulk material that can leave the exposed surface brittle and lead to cracking, crack linkage and gross material loss.
Typically, components that are exposed to abrasive flows are subject to various hard facing treatments to improve abrasion resistance. Such treatments often include either surface preparations that harden and smooth the base material itself or bonding abrasion-resistant materials to the surface of the base material. Surface preparations can often make the base material more resistant to impact from particles with low kinetic energy, but these same preparations can leave the base material more brittle and thus susceptible to cracking as a result of impacts from high kinetic energy particles. Bonding of abrasion-resistant materials is typically performed using thermal spray techniques such as High Velocity Oxy-Fuel (HVOF) or Air Plasma Spray (APS). However, in highly abrasive environments, the residual tensile stress that results from multiple impacts can accumulate at the junction of a base material and its bonded coating, leading to delamination of the coating material.
Because of the harshly abrasive environment of wellbore operations, significant effort and expense is expended to mitigate abrasive loss and improve wellbore tool and equipment life. Hard facing treatments, as described above, are used extensively to protect a wide array of wellbore tools. Also, wellbore tools and equipment are often over-designed to provide adequate service life. However, all of these steps routinely prove inadequate to provide sufficient protection from abrasion, and wellbore operations are often interrupted to replace broken tools that were unable to withstand the prolonged stress.
Another recurrent issue with the use of abrasive slurries is that pumping them requires a considerable expenditure of energy. The same mechanisms that lead to abrasive wear also lead to resistance to flow in the form of friction between the slurry and the piping used to transfer the slurry from the surface to the formation. If this friction could be reduced, more of the surface pressure would be transferred to the formation, leading to more efficient fracturing operations. The polymers used in fracturing and friction reducers added to the fracturing fluid formulations could minimize this effect.
From the foregoing it will be apparent that there is a need for an improved method of placing solids in a wellbore that does not expose wellbore equipment to the abrasive effects of slurry flow of material and permits greater pumping and fracturing efficiency.